Diagnosis

A thorough exam and appropriate diagnostic tests are essential for an accurate diagnosis. A misdiagnosed penicillin allergy may result in the use of less-appropriate or more-expensive antibiotics.

Your doctor will conduct a physical examination, ask questions about your symptoms and order additional tests. You may be referred to an allergy specialist (allergist) for these tests. These may include the following.

Skin tests

With a skin test, the allergist or nurse administers a small amount of the suspect penicillin to your skin with a tiny needle. A positive reaction to a test will cause a red, itchy, raised bump.

A positive result indicates a high likelihood of penicillin allergy. A negative test result usually means you're not at high risk of an allergy to penicillin. But a negative result is more difficult to interpret because some kinds of drug reactions cannot be detected by skin tests.

Graded challenge

If the diagnosis of a penicillin allergy is uncertain, a graded drug challenge may be recommended. With this procedure, you receive up to five doses of the suspect penicillin, starting with a small dose and increasing to the desired dose. If you reach the therapeutic dose with no reaction, then your doctor will conclude you aren't allergic to that type of penicillin. You will be able to take the drug as prescribed.

If you are allergic to one type of penicillin, your doctor may recommend a graded challenge with a type of penicillin or cephalosporin that's less likely — because of known chemical properties — to cause an allergic reaction. This would enable your doctor to identify an antibiotic that can be used safely.

During a drug challenge, your doctor provides careful supervision, and supportive care services are available to treat an adverse reaction.